It has been a long sought after goal to control the compaction of soil, road base, bituminous and similar materials by a vibratory compactor so that the desired material density is achieved in as few passes by the compactor as possible. Often, to assure adequate compaction, a compactor operator continues to work the material after it has achieved the desired density, which may over compact the material. This practice is wasteful of both time and equipment. It has also been recognized that it would be highly desirable to continuously monitor compaction operations, while they are being performed, to assure that the required material density is being achieved uniformly across the entire work area.
Several devices, using one or more parameters relating to a characteristic of soil density, have been proposed for controlling the vibratory compaction of soil and roadway materials. For example, West German Patent No. 2,942,334 issued Jun. 28, 1984 to Koehring Bomag GMBH describes an apparatus for monitoring the degree of compaction by measuring the value of an operating parameter, such as the hydraulic pressure in the vehicle drive system, and comparing the measured value with values measured during preceding passes of the vehicle.
Other devices measure selected physical characteristics of the vehicle which vary with the density of the compactible material. Kerridge U.S. Pat. No. 3,599,543 issued Aug. 17, 1971 compares the length of the major axis of the elliptical path of a point on the vehicle's vibrating roller with the length of the axis when the ground is fully compacted. Swedish Patent No. 76 08709, published Feb. 27, 1978 to Heinz Thurner measures the amplitude of the vertical motion of the vibrating roller at a fundamental frequency and at one or more harmonic frequencies, and calculates a ratio of the measured fundamental and harmonic frequencies. Swedish Patent No. 80 08299 published Jun. 28, 1982 to Geodynamic Thurner AB et al, relates the degree of compaction to the shape of a waveform representative of the vertical movement of the vibratory compactor.
The values of the parameters measured by the above described devices and methods are sensitive to the rotational frequency and mass of the eccentrically mounted member and, in some instances, to vehicle speed. Consequently, in order to obtain comparable data, i.e., data that can be directly correlated with previously or subsequently recorded data, or with other predetermined values to evaluate the present degree of compaction, the frequency and mass of the eccentrically mounted member, the vehicle speed and other such operating conditions that influence the value of the measured parameter must be maintained uniformly throughout a particular compaction operation.
This requirement often prevents use of a vibratory compactor in the most efficient manner. It is often desirable to change the ground speed of the vehicle, or the mass or frequency of the eccentrically mounted rotating member during a specific compaction operation. For example, the resonant frequency, i.e., the frequency at which the amplitude of the material contacting tool, or drum, has a maximum value, is influenced by material density. Adjustment of the rotational frequency of the eccentrically mounted mass to maintain operation at the resonant frequency throughout the compaction operation is therefore desirable. The frequency adjustment may be carried out automatically by a closed loop system, or manually by an operator. Examples of frequency control systems based on the angular relationship of the respective positions of the rotating eccentric mass and the vibrating drum are described in French Patent No. 2,390,546 issued Jan. 12, 1979 to Albaret S. A. and in Jesse W. Harris U.S. Pat. No. 3,797,954 issued Mar. 19, 1974.
Thus, material density correlating devices and methods, such as those described above, which rely on the value of measured parameters which are sensitive to changes in operating conditions, are unsuitable for use in compacting operations in which it is desirable to vary the operating characteristics of the vehicle.
Furthermore, the sensitivity of density correlation methods based on some resonant property of the physical system comprising the vibratory vehicle and the compactible material, such as the devices and methods described above, decreases as material density increases. Therefore, it becomes increasingly difficult to detect small changes in the density of the compactible material as the amount of compaction approaches the desired value. This characteristic, when combined with the requirement to maintain uniform operation of the vehicle throughout the compacting operation, makes the above devices impractical to use.
The present invention is directed to overcoming the problems set forth above. It is desirable to have an apparatus for increasing the density of a compactible material that is able to continuously and accurately evaluate the increase in material density. It is also desirable to have a method for continuously evaluating material density that is particularly sensitive to small changes in density as the material density approaches the desired value.
By way of comparison, a number of tests were made at the Centre d'Experimentations Routieres, Rouen, France, comparing currently used density correlation methods with the method of the present invention. The tests were all carried out by the same vibratory compactor on crushed gravel material designated in French Listes d'Aptitude as Material D3. This material, a material commonly used in road and highway construction, is considered to be very difficult to compact. The density of the D3 material was measured after the 2nd, 10th, 20th, 30th and 64th pass over the material by the vibratory compactor. The percent change of the measured parameters of the vibratory compactor was also calculated after the same number of passes over the material. The parameters measured were the hydraulic pressure in the vehicle drive circuit, the vertical acceleration of the drum, the harmonic ratio of the vertical acceleration of the drum, and the harmonic ratio of the vertical amplitude of the drum. In addition, the values determined by the method of the present invention which continuously calculates the total force applied by the material contacting member is identified as TAF (Total Applied Force) in the following table.
______________________________________ PERCENT CHANGE OF MEASURED OR CALCULATED PARAMETER COMPARED TO ACTUAL PERCENT CHANGE OF SOIL DENSITY NUMBER OF PASSES PARAMETER 2 .fwdarw. 10 2 .fwdarw. 20 2 .fwdarw. 30 2 .fwdarw. 64 ______________________________________ Actual Density +3.3% +4.73% +5.57% +7.10% Hydraulic Pres- -17.5% -25.2% -29.7% -38.0% sure Propel Circuit Vertical Accel- +11.6% +16.5% +19.5% +24.9% eration (Drum) Harmonic Ratio +16.8% +24.0% +28.3% +36.2% Vertical Acceleration Harmonic Ratio +21.0% +30.2% +35.5% +36.2% Amplitude TAF +95.3% +136.4% +160.4% +205.3% (Total Applied Force) ______________________________________
As can be seen, the parameter identified as TAF is particularly sensitive to small increases in material density.